Public Good vs Positive Externality
Public Good and Positive Externality are two Market Failure & Government concepts in AP Economics that students often mix up. A public good is non-excludable and non-rival: no one can be excluded from it, and one person's use does not reduce another's. A positive externality is a benefit enjoyed by a third party not involved in a transaction, such as vaccination or education. Here is how they compare side by side.
National defense and street lighting are classic examples. Because users cannot be excluded, markets underprovide public goods due to the free-rider problem. They are usually funded by government through taxation.
Because buyers ignore these external benefits, the market underproduces relative to the socially optimal quantity. The marginal social benefit exceeds the marginal private benefit. Governments correct it with subsidies or public provision.
Public Good vs Positive Externality: Two Different Reasons Markets Underprovide
| Public Good | Positive Externality | |
|---|---|---|
| What the term describes | A type of good, defined by its characteristics | A spillover benefit created by a transaction |
| Who receives the benefit | Everyone, and nobody can be shut out | The buyer, plus a third party outside the deal |
| How much the market supplies | Often close to nothing, because of free riding | Some, but less than the efficient quantity |
| Diagram you draw | Individual demands added vertically against marginal cost | Marginal social benefit sitting above marginal private benefit |
| Standard remedy | Government provision funded through taxes | A per-unit subsidy equal to the external benefit |
| Test to apply | Is it non-rival and non-excludable | Does somebody outside the transaction gain without paying |
| Examples | National defense, a flood levee, a lighthouse | Vaccination, education, a restored building front |
A flu shot is a private good with a positive externality, not a public good
The two ideas answer different questions, and a vaccination clinic shows why. A flu shot is excludable, because the clinic can turn away anyone who does not pay, and it is rival, because a dose given to you cannot also be given to me. By the definitions that makes it a private good. It also protects people you will never meet, since a vaccinated person is less likely to pass the virus along, and that is a positive externality. Put numbers on it. Suppose private demand is P = 40 - Q and marginal cost is P = 10 + Q, with Q measured in millions of doses. The market settles where 40 - Q equals 10 + Q, which gives 15 million doses at a price of $25. Now add an external benefit of $10 per dose. Marginal social benefit becomes 50 - Q, and the efficient quantity solves 50 - Q = 10 + Q, giving 20 million doses. The market falls 5 million doses short, and the value society loses is the triangle between the two benefit lines across that gap, half of $10 times 5 million, or $25 million. A subsidy of $10 per dose closes it exactly. The steps behind that calculation are laid out at /calculate/socially-optimal-quantity.
Free riding breaks the market; a spillover only bends it
That difference in severity is the one worth carrying into an exam. With a positive externality the market still functions. Doses are sold, sellers collect revenue, and the only complaint is that the quantity is too small, which a subsidy corrects without anyone leaving the private sector. A public good is a harsher case, because non-excludability means the seller cannot collect from anybody at all. Suppose three households value a streetlight at $40, $30 and $20 a year. The light is worth $90 to them against a cost of $75, so it clearly should be installed. No household will pay $75 alone, and each has a reason to wait for one of the others, since the light shines on all three regardless of who paid. Private provision can therefore stall at zero even when the benefit plainly exceeds the cost, which is the /glossary/free-rider-problem in one paragraph. Two consequences follow. Demand for a public good is added vertically, because every household consumes the same unit at once, while demand for a private good is added horizontally. And the usual remedy is public provision financed by taxes, because a subsidy only helps a seller who can already charge somebody.
Frequently asked questions
Is a positive externality the same as a public good?
No, a positive externality is a benefit landing on a third party outside a transaction, while a public good is a good that is non-rival and non-excludable. Public goods do generate benefits nobody pays for, but plenty of ordinary private goods carry positive externalities too.
Is education a public good?
No, education is excludable and rival at the level of a classroom seat, which makes it a private good carrying a large positive externality. That spillover, rather than the definition of a public good, is the reason governments subsidize schooling or supply it directly.
Why do governments provide public goods instead of subsidizing them?
Because a subsidy only helps a seller who can already charge buyers, and non-excludability means nobody can be made to pay for a public good in the first place. With no revenue stream to top up, the government usually has to buy or produce the good and finance it through taxation.
Live Externalities graph. Drag the curves, or open the full version.
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